The characteristics of two distinct suppressor cell populations, i.e., antigen-specific and idiotype-specific, and their role in regulating the immune response to phosphorylcholine (PC) will be investigated using an in vitro culture system. The antigen-specific and idiotype-specific suppressor cells are induced, respectively, either by a neonatal injection of PC-containing pneumococcal C-polysaccharide or by injection of antibodies to the antibody-combining sites (idiotype) of anti-PC. The biological properties of these suppressor cells as well as their recognition sites for target cells will be characterized using various serological reagents. In addition, the effect of interactions between these classes of suppressor cells on anti-PC production by normal splenic B cells will be explored employing mixtures of the two supressor cell populations in vitro. Finally, the mechanisms involved in the generation of idiotype-specific suppressor cells in vitro will be investigated with an emphasis on the requirements for induction and cell types participated.